Electrical Transmission and Grounding
Date: Spring 2012
NOTE: Normally we do not except multiple questions even on the same topic. These series of questions are closely related and the responding scientist has chosen to answer them sparately. The individula questions are followed by their answers.
I have the following electric Power grid questions:
Utility poles have a grounded wire. Is this just for lightening?
------------Lightning cables are about a quarter inch in diameter and are sometimes attached to utility poles.
Sometimes a small cable may be a ground wire for some auxiliary service like fiber optic cable or telephone.
Is not connected the neither one of the 3 phase wires, is it?
-----------You have to look at the area to see if it is connected.
Depending on what the cable is for, I doubt you would find it connected to one of the three phase wires
Or else it would not be at ground level where people could touch it or power could leak to ground.
What does the multi-grounded neutral wire do, and has it always have to be present?
----------It serves as the ground reference potential. If there is not a ground wire accompanying the three phase power lines, then the power company is depending on there being no potential difference between the ground at the transmission site and the customer’s site.
The 3 phase wires are only grounded at destination when electricity is being consumed?
----------The 3 phase wires are not grounded at the destination.
If they were grounded at the destination then what is the use of transmitting the power anyway?
-----------The customer takes the high voltage lines and draws current from them to do work (through a load) while running to ground.
If domestic consumers receive just one phase current, why there are 3 wires coming from the pole to the house?
----------The three wires on the poles carry high voltage in three phases at 120 degrees apart.
This is done to transmit high levels of power without a corresponding high level of current because high levels of current burn up the transmission lines.
So as you go down the power lines, successive transformers step down the voltage from different wires of the 3 phase source to distribute the power at sustainable current levels.
As you go down the line the first transformer will draw power from phase A, the 2nd from phase B, the 3rd from phase C, the 4th from phase A, the 5th from phase B, and so on. This technique balances the current load on all three phases, if done right.
Downstream, transformers sequentially step down the power line voltage to household voltage level of 115V.
Households connect loads to (typically) 115 volts which draw current. The metered parameter is current.
That is, all (well mostly all) households have 115 volts delivered to their house by the electric power company. By providing the same voltage to all households, the power company need only measure the current each household draws to derive the amount of power the household uses for the bill.
For instance, all household light bulbs (or mostly all) are rated at 115 V.
A 60 Watt bulb will draw (60/115) 0.5217 Amps.
A 120 Watt bulb will draw (120/115) 1.0435 Amps.
So current is used as the parameter to measure how many Watts a household uses.
The meter at your house only measures the current you use.
Is ground always the return path for electrons?
----------Ground is the voltage reference point from which the power medium (electrons) is elevated to a higher level of energy.
Destination ground is the point where the power medium tries to restore to and drops off its energy in the device through which it must pass to reach ground.
Sometimes there is a current in the ground circuit to neutralize the potential differences in the ground circuit that develop from time to time.
There is no need for a return path for electrons if the ground at the transmission site is at the same potential difference (voltage) at the customer site, but sometimes a path must be created between the two grounds to assure they remain at the same reference level.
Is there a ground connection at every house directly or at the nearest utility pole?
----------There is a (usually) rod driven into the ground at your house, somewhere near the meter.
The power line brings in the voltage and current (power) into your house.
You add a load to it to draw a current to give the power a path to ground. While going to ground, the power does work.
The ground at your house serves as a local low voltage point toward which your current throws itself.
In power plants the wires in the generator are connected to the ground, to close the circuit?
----------If you did that, you would short circuit the generators. That is called a crow bar.
Look at the generator as a device that provides charges at a potential difference (Voltage) above a ground reference level.
It transforms chemical (diesel fuel) or some other energy source (hydro) to electrical energy.
The ground that all the little charges the generator creates rush to the ground on your coffee maker at your house.
The ground potential at the transmission site must (ideally) be the same as at the customer site.
Google “Kirchoff’s Voltage law” to see how the sum of all voltage rises (generators) and voltage drops across households (light bulbs) sum to zero in the loop of the circuit.
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Update: June 2012